Two recent advances have altered the understanding of the small nucleolar RNAs ("snoRNA"s) in eukaryotic cells. The first was the discovery that nearly all snoRNAs can be classified into two large families, based on short conserved sequence elements (Balakin et al., Cell, 86:823-834, 1996). The families are known as the box C/D and box ACA families. Only one known snoRNA does not belong to either family. This is the phylogenetically conserved MRP/7-2 snoRNA, which is part of a snoRNP complex involved in rRNA processing (Maxwell et al., Annu. Rev. Biochem., 35:897-934, 1995).
Members of the box C/D family contain box C and box D elements that are almost always near the 5' and 3' ends of the snoRNA molecule. These elements have been implicated the synthesis, localization, and function of snoRNAs (see, e.g., Maxwell et al., supra; and Huang et al., Mol. Cell. Biol., 12:4456-4463, 1992).
The box ACA family includes snoRNAs with an ACA triplet (or certain active variants thereof) located three nucleotides upstream from the 3' terminus (Balakin et al., Cell, 86:823-834, 1996). These snoRNAs are also referred to as "ACA snoRNAs" or "ACA box snoRNAs."